The present invention relates to the structure of a capacitor used in a semiconductor integrated circuit. More specifically, the invention relates to a capacitor which employs a ferroelectric material, and a nonvolatile memory using this type of capacitor.
A memory cell disclosed in Japanese Patent Application Unexamined Publication No. Hei. 2-304796 is an example of this type of nonvolatile memory. FIGS. 6 and 7 show an equivalent circuit and a structure of this memory cell, respectively.
As shown in FIG. 6, the memory cell includes a field-effect transistor 10 as a switching device and a signal charge storing capacitor 20 using a ferroelectric material. The field-effect transistor 10 has a gate electrode 11, a drain electrode 12 and a source electrode 13, and the gate electrode 11 and the drain electrode 12 are connected to a word line L and a bit line BL, respectively. The capacitor 20 has a structure in which a ferroelectric thin-film 23 is sandwiched between two opposed electrodes 21 and 22. The electrodes 21 and 22 are respectively connected to the source electrode 13 of the field-effect transistor 10 and a ground line V.sub.SS or a drive line DL. The ferroelectric thin-film 23 is made of, for instance, lead zirconate titanate (PZT).
Referring to FIG. 7, the structure of the above memory cell is described in more detail. A field oxide film 2 is formed on a silicon substrate 1 by selectively oxidizing its surface layer to separate an area in which the devices are to be formed. A field-effect transistor 10 consisting of a gate electrode 11 covered with an oxide film 3, a drain region 12a and a source region 13a is formed in this separated area. A first electrode 21, a ferroelectric thin-film 23 and a second electrode 22 are sequentially laid in this order on the source region 13a, to form a capacitor 20. Then, a metal lead 4 for the bit line BL is formed on the drain region 12a, and another metal lead 5 for the ground line V.sub.SS or drive line DL is formed on the second electrode 22.
However, the conventional memory cell having the above structure is associated with the following problem. Since ferroelectric materials such as PZT have a large dielectric constant, they are suitable for the charge storing material in the above memory cell. Due to the requirement of high-density integration, the cell area of the above type of memory cell and, therefore, the area of the capacitor should be designed to be as small as possible. However, there is some limit in reducing the capacitor area, because the decreased signal charge quantity stored in the capacitor, as is caused by the excessive reduction of the capacitor area, will result in a difficulty in the signal reading.